Electrophoretic NMR has been applied for the first time to measure electroosmotic drag coefficients Kdrag in polymer electrolyte membranes. Theoretical and experimental details of the method are discussed and measurements of Kdrag as a function of water content and temperature are reported for Nafion® 117 and sulfonated PEEKK. For a given water content n=[H2O]/[SO3H], the values for sulfonated PEEKK are lower than for Nafion, but for the water contents of the highest proton conductivities observed (n=20 for Nafion n=40 for sulfonated PEEKK), the results are similar for both polymers (Kdrag=2.6 for Nafion and Kdrag=3.1 for sulfonated PEEKK). A hydrodynamic model equation with the rate of proton transfer processes and the water–polymer interaction as parameters is used for the interpretation of the data.